Harold Hall

Workshop Processes

The illustrations are of course exaggerated but it will be found in practice that
the part can move within the chucks jaws by as much as 3mm within a few minute's
running, something I can confirm from experience. Even quicker if the error is substantial.
It is essential therefore that the lathe operator fully appreciates this situation
and takes the necessary steps to avoid it. Of course the effect diminishes the further
the steady is from the chuck and with a smaller diameter being held. Conversely,
the effect is much greater with diameters that can only be held in the chucks reverse
jaws which have a much shallower grip.

However, even at greater distances from the chuck, say 200mm working at 25mm diameter,
where the machine, steady and workpiece may deflect sufficient for the situation
not to occur, considerable stress will be being placed onto the lathe's bearings
and the steady's arms and should be avoided. Also, the pressure being placed on the
steady's arms will result in premature wear and a considerable amount of heat being
developed.

Unfortunately, it is impossible to define at what lengths and diameters the problem
diminishes to acceptable limits, maybe 8mm diameter at 100mm distance or 20mm diameter
at 300mm and it is best avoided therefore unless the user of the lathe is confident
that all will be well. How then can this be achieved? The simple answer is to use
the four jaw chuck and set the workpiece to run true, this though is too simplistic.

Most lathe owners will understand that if a workpiece is set to run true adjacent
to the jaws of a four jaw chuck it is probable that if tested at a distance from
the jaws that an error will exist. This due to the poor state of the jaws or perhaps
the material itself being very slightly bent. This situation can become extreme if
a large diameter workpiece is having to be held in the chuck's reverse jaws as the
depth of the grip will be insufficient to pull the workpiece into line.

The extremes

Unfortunately, there is not just one way of applying the steady to the workpiece
as the workpieces themselves will vary considerably, I will therefore first discuss
the extremes.

If the material is short and of a large diameter there is no possibility of it bending
to compensate for differences between it and the setting of the steady. In this
case therefore the four jaw chuck must be used to set the material to run concentric
with the lathe's spindle at the point that the steady is to positioned. Because of
this the material in unlikely to be running true at its jaws but in many cases this
will be of no importance.